A synthesis parameter of molten salt nanofluids for solar thermal energy storage applications

Nanoparticle dispersions at a minute concentration in salt mixtures have recently been reported to significantly increase their specific heat capacity. However, there is a discrepancy between experimental results reported for the same kind of molten salt-nanoparticle mixtures by different research groups. From our literature survey, we learned that there is no standardized synthesis protocol for preparing molten salt nanofluids. In general, two-step method has been the most widely used in the literature. Nanoparticles and salts are dispersed in an aqueous solution. The mixture is then ultrasonicated to disperse the nanoparticles homogeneously. The mixture is then dried at elevated temperatures to obtain uniformly mixed salt-nanoparticle mixture, which results in a molten salt nanofluid when the mixture is heated above the melting point of the salt mixture. In this method, the duration of the ultrasonication can play an important role in the dispersion of nanoparticles. However, there is no consensus on how much time is required to disperse nanoparticles homogeneously. Literature survey shows that the duration of ultrasonication used to prepare molten salt nanofluids varies from 0.5 h to 5 h. In this study, we assume that the discrepancy reported in the literature could be due to different ultrasonication times used in different studies. We chose the most widely studied molten salt nanofluids, synthesized in different ultrasonication times, compared their specific heat capacity enhancements, and proposed the optimum ultrasonication time to apply for future molten salt nanofluid preparation. Thermogravimetric analysis was performed to study the thermal stability of the pure salts and nanofluids from 100 °C to 600 °C. Nanofluids prepared using 3 h of ultrasonication time showed the highest enhancement of 1.70 kJ/kg°C, which was approximately 15.6 % higher than the pure base salts. The result of the study can be impactful in solar energy industry sector and can be used as a guideline to prepare molten salt nanofluids for their solar thermal energy storage media.